(1) Field of the Invention
The present invention relates to a technique for diagnosing whether or not optical lines and optical transmission apparatuses operate normally, in an optical ring network having a redundant configuration including an active system and a standby system. More specifically, the present invention relates to a method of diagnosing a state of the standby system during operation of the active system, and an optical ring network using the same.
(2) Description of the Related Art
For example, in an optical transmission apparatus corresponding to, such as, a synchronous optical network (SONET), which is known as one of the high-speed digital communication systems using optical fibers, line relief at the time of failure occurrence can be performed instantaneously, by using a redundant configuration including an active system and a standby system to transmit an optical signal.
The redundant configuration in an optical network constructed using such optical transmission apparatuses, has two main meanings. One is the redundancy of an optical fiber transmission line itself, and the other is the redundancy of a signal processor in the optical transmission apparatus. In the optical network having the redundant configuration, it is important to always confirm that the optical line and the optical transmission apparatus in the standby system are in normal states, in order to prepare for a situation in which a failure occurs in the optical line or the optical transmission apparatus in the active system.
However, in the conventional optical network, there is not provided a function for diagnosing the state of the standby system during operation of the active system. Therefore, after a failure has occurred in the active system, a relief function is activated on the standby system, and then, the states of the optical line and the optical transmission apparatus in the standby system are confirmed. As a result, even if the switching over from the active system to the standby system is performed, if a failure such as disconnection or breakdown has occurred in the standby system, an optical channel under operation cannot be actually relieved, thereby causing a problem.
In order to deal with such a problem, for example, a technique has been proposed for the optical transmission apparatus having the redundant configuration, in which at the time of confirming the state of the standby system, test data is sent to the standby system without affecting the communication with the active system, to confirm the normal operation of the standby system without switching between the active system and the standby system (refer to Japanese Unexamined Patent Publication No. 7-15757).
However, the diagnosis of the standby system in the conventional technique as mentioned above is performed in a state where the standby system is closed in each optical transmission apparatus constituting the optical network. Therefore, it is difficult to confirm the normal states of the optical line and the optical transmission apparatus in the standby system for the whole optical network. Particularly, in a ring type optical network such as a bidirectional line switched ring (BLSR) supported by the aforementioned SONET, if the normal state of the standby system is not confirmed at the level of the whole network, it is difficult to reliably relieve the optical channel under operation, at the time of failure occurrence in the active system.
Further, for example, in the case of an optical network employing a protection channel access (PCA) system which also passes data signals through the optical line in the standby system during the operation of the active system, to increase the transmission capacity, it is not easy to detect a failure in the standby system without interrupting the data signals transmitted through the PCA circuit during the normal operation. Therefore, the detection of failure in the standby system becomes more difficult.